1. Field of the Invention
The present invention relates generally to a microscope and, more particularly, to an apparatus for controlling revolutions of a motor-driven revolver mounted with objective lenses.
2. Related Background Art
A revolver of a microscope generally has a plurality of objective lenses having different magnifications mounted thereon, which are disposed at fixed intervals on the circumference thereof in the sequence of the magnifications. As illustrated in FIG. 6, an objective lens L1 exhibiting a high magnification has a long lens barrel and a small focal depth D1. An objective lens L2 exhibiting a low magnification has a short lens barrel and a large focal depth D2.
For this reason, in a state such as observing a specimen in close proximity thereto by use of the objective lens L2 exhibiting low magnification and large focal depth, i.e., in an as-observed state where the specimen is disposed in a position P1 closer to the objective lens L2 than a focal plane FP of the objective lens, when the objective lens is abruptly switched over to the high-magnification objective lens, the front edge of the lens barrel of the objective lens may collide with the specimen. As a result, the lens is damaged, or the specimen, e.g., a wafer, is damaged.
More specifically, for example, addresses 1-5 are set sequentially in a clockwise direction in five objective lens mounting portions provided in a revolver. The objective lenses are mounted in these mounting portions in the sequence of the magnifications. That is, the objective lens having a minimum magnification is mounted in an address-1 mounting portion, while the objective lens having a maximum magnification is mounted in an address-5 mounting portion. When observing through the minimum-magnification objective lens if an operator carelessly operates to make a counterclockwise movement to change an observing magnification, it may happen that the front edge of the maximum-magnification lens collides with the specimen.
As explained above, in the conventional microscope, when the microscope operator carelessly gives a command to switch over the objective lens from the low-magnification objective lens to the high-magnification objective lens, the maximum-magnification objective lens collides with the specimen, with the result that the objective lens and the specimen are damaged. This happens particularly when the objective lens mounted in the revolver is changed from the minimum-magnification objective lens to the maximum-magnification objective lens. Accordingly, when changing the observing magnification (to a higher-magnification), the microscope operator needs to revolve the revolver while paying attention to this.